Fibre-containing composite materials such as CFRP and GFRP represent a particular challenge, especially in terms of good hole quality. These materials are common materials for aircraft structure. These composite materials, particularly CFRP with twill fibre, unidirectional fibre layout, and materials with a glass cloth on the exit face, are notoriously difficult to drill and poor exit hole quality is the norm with conventional drills.
Some attempts have been made to address these drawbacks. In particular the present inventors suggested that a twist drill having a variable helix can be used to drill composite materials such as CFRP and GFRP to produce good exit hole quality (PCT/GB2011/000478). However, a twist drill having a variable helix can be complex to manufacture.
Typical tool life is 60 holes and so a considerable number of twist drills are used in the manufacture of aircraft and the like. Whilst hole quality is important such that comparatively complex geometry can be favoured if they perform well, there is nevertheless a desire to reduce costs where possible and to ensure a ready and rapid supply of replacement twist drills. Thus, improvements in tool life and/or unit cost are of interest.
Manufacturing drills with complicated geometries, like variable helix, require not only new modelling techniques but also new manufacturing methodology.
WO 2008/013725 (Kennametal) aims to provide a twist drill for drilling CFRP, with good exit hole quality and good tool life. WO 2008/013725 proposes a diamond coated twist drill made of tungsten carbide with 6 wt % cobalt, having a narrowly defined geometry including lip relief angle between 10° and 20°, a notch rake angle between −5° and 10°, a chisel length up to 0.035 mm and a point angle between 70° and 110°. Kennametal teaches that these features in combination provide a coated twist drill with good exit hole quality and good tool life.